Method and apparatus for modifying the effects of color burst modifications to a video signal

ABSTRACT

In the known color stripe process for preventing recording of video signals, the color burst present on each line of active video is modified so that any subsequent video tape recording of the video signal shows variations in the color fidelity that appear as undesirable bands or stripes of color error. This color stripe process is defeated first by determining the location of the video lines including the color stripe process, either by prior experimentation or by on-line detection. Then some or all of the lines including the modified color bursts are modified so as to render the overall video signal recordable. The modification is accomplished in a number of ways, including phase shifting the color stripe burst into the correct phase, replacing some of the color stripe bursts or a portion of particular color stripe bursts so that they are no longer effective, and mixing the color stripe burst with color stripe signals of the correct phase so as to eliminate most or all of the phase error present. The modified color bursts are defeated, in other versions, by modifying the horizontal sync pulse signals immediately preceding the modified color bursts so that the modified color bursts are not detected by a VCR and hence have no effect.

This application is a continuation of application Ser. No. 09/042,680filed Mar. 12, 1998, U.S. Pat. No. 6,327,422, which in turn is adivision of application Ser. No. 08/897,132 filed Jul. 18, 1997, U.S.Pat. No. 5,784,523, which is a continuation of application Ser. No.08/438,155 filed May 9, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a method and apparatus for processinga video signal, and more particularly to removing (defeating) effects ofphase modulation of the color burst component of the video signal.

2. Description of the Prior Art

U.S. Pat. No. 4,577,216, “Method and Apparatus For Processing a VideoSignal,” John O. Ryan, issued Mar. 18, 1986 and incorporated byreference, discloses modifying a color video signal to inhibit themaking of acceptable video recordings thereof. A conventional televisionreceiver produces a normal color picture from the modified signal.However, the resultant color picture from a subsequent video taperecording shows variations in the color fidelity that appear as bands orstripes of color error. Colloquially the modifications are called the“color stripe system” or the “color stripe process”. Commercialembodiments of the teachings of this patent typically limit the numberof video lines per field having the induced color error or colorstripes.

Color video signals (both in the NTSC and PAL TV systems) include whatis called a color burst. The color stripe system modifies the colorburst. The suppression of the color subcarrier signal at the TVtransmitter requires that the color TV receiver include (in NTSC) a 3.58MHz oscillator which is used during demodulation to reinsert the colorsubcarrier signal and restore the color signal to its original form.Both the frequency and phase of this reinserted subcarrier signal arecritical for color reproduction. Therefore, it is necessary tosynchronize the color TV receiver's local 3.58 MHz oscillator so thatits frequency and phase are in step with the subcarrier signal at thetransmitter.

This synchronization is accomplished by transmitting a small sample ofthe transmitter's 3.58 MHz subcarrier signal during the back porchinterval of the horizontal blanking pulse. FIG. 1A shows one horizontalblanking interval for color TV. The horizontal sync pulse, the frontporch and blanking interval duration are essentially the same as thatfor black and white TV. However, during color TV transmission (bothbroadcast and cable) 8 to 10 cycles of the 3.58 MHz subcarrier that isto be used as the color sync signal are superimposed on the back porch.This color sync signal is referred to as the “color burst” or “burst”.The color burst peak-to-peak amplitude (40 IRE for NTSC TV as shown) isthe same amplitude as the horizontal sync pulse.

FIG. 1B shows an expanded view of a part of the waveform of FIG. 1Aincluding the actual color burst cycles. During the color TV blankingintervals, such a color burst is transmitted following each horizontalsync pulse.

In one commercial embodiment of the color stripe process, no color burstphase (stripe) modification appears in the video lines that have a colorburst signal during the vertical blanking interval. These are lines 10to 21 in an NTSC signal and corresponding lines in a PAL signal. Thecolor stripe modifications occur in bands of four to five video lines ofthe viewable TV field followed by bands of eight to ten video lineswithout the color stripe modulation. The location of the bands is fixed(“stationary”) field-to-field. This color stripe process has been foundto be quite effective for cable television, especially when combinedwith the teachings of U.S. Pat. No. 4,631,603 also invented by John O.Ryan and incorporated herein by reference.

In NTSC TV, the start of color burst is defined by the zero-crossing(positive or negative slope) that precedes the first half cycle ofsubcarrier (color burst) that is 50% or greater of the color burstamplitude. It is to be understood that the color stripe process shiftsthe phase of the color burst cycles relative to their nominal (correct)position which is shown in FIG. 1B. The phase shifted color burst isshown in FIG. 1C. The amount of phase shift shown in FIG. 1C is 180°(the maximum possible).

Further, the amount of phase shift in the color stripe process can varyfrom e.g. 20° to 180°; the more phase shift, the greater the visualeffect in terms of color shift. In a color stripe process for PAL TV, asomewhat greater phase shift (e.g. 40° to 180°) is used to be effective.

Other variations of the color stripe process are also possible.

U.S. Pat. No. 4,626,890, “Method and Apparatus For Removing PhaseModulation From the Color Burst”, John O. Ryan, issued Dec. 2, 1986 andincorporated by reference, discloses removing the phase modulation ofthe U.S. Pat. No. 4,577,216. This removal is useful in eliminating muchof the effects of the process disclosed in U.S. Pat. No. 4,577,216 forrecording.

SUMMARY

The present inventors have determined that improvements are possible onthe teachings of above mentioned U.S. Pat. No. 4,626,890, especiallypertaining to eliminating or reducing the effects of certain variants asdescribed above of the color stripe process of U.S. Pat. No. 4,577,216.

Thus in accordance with the present invention, a circuit modifies and/orremoves the color stripe process, or modifies the video signal so thecolor stripe process is not evident, i.e. has no influence on atelevision set or VCR.

In one embodiment, the video line locations of the color stripe colorbursts are known. That is, it is known in which video lines the colorstripe modified bursts occur, as in the above described commercialembodiment of the color stripe process. These locations are stored in apreprogrammed memory which provides signals indicating those videolines. Also, the same preprogrammed memory provides an indication ofwhether the entire color stripe burst or only a part of it is to bemodified.

A modification circuit which also receives the video signal, and usesthe information as to the location of the color stripe bursts, removesand/or modifies the color stripe bursts or otherwise modifies the videosignal (i.e. modifies the horizontal sync pulse immediately precedingthe color stripe burst) so that the effect of the color stripe processis attenuated or eliminated.

With regard to the present invention, it has been found that it is notnecessary to completely eliminate the color stripe bursts; with typicalcommercially available television sets and VCRs, eliminating some of thecolor stripe bursts or attenuating the color stripe bursts either interms of amplitude or duration, or removing or attenuating a portion ofeach or most colorstripe bursts, has been found effective to overcomethe effect of the color stripe process, allowing a recordable (copiable)video signal to be produced.

Sometimes the color stripe process is not fixed in line location. Othertimes, even where it is so fixed, it is not desired or possible toprovide the preprogrammed memory. Then instead a phase detector detects,for each video line, the presence of a color stripe burst, i.e. detectscolor bursts having induced phase modulation. Upon detection of thecolor stripe burst, the modification circuit (as above) modifies eitherthe color burst or other portions of the video signal (i.e. thehorizontal sync pulse) so as to attenuate or eliminate the effect of thecolor stripe burst.

It is to be understood that correcting or replacing the color stripebursts in accordance with the invention does not require completelyeliminating the phase shift (modulation); a reduction of the phase shiftto some small value (e.g. 5° or less for NTSC) has been found to beeffective, in that the typical viewer will not perceive the attendantcolor shift.

Thus, the present method and apparatus have several embodiments. Thereare several different methods of determining the location of the colorstripe burst, either by knowing its location from prior analysis or byactual detection. Various embodiments are also disclosed herein fordefeating the color stripe process. These, as described above, generallyrely on first determining the video line locations of the color stripebursts, either by knowing their location from prior analysis andprogramming in the location to e.g. a programmable memory, or by sensingeach individual color stripe burst color burst on a video line-by-linebasis. Either of these methods for determining the location of the colorstripe burst may be achieved by various circuits.

As an alternative, one replaces all color bursts with correct phasebursts, generating the correct bursts by particular circuits differentfrom that illustrated in Ryan U.S. Pat. No. 4,626,890 as disclosedbelow.

A typical circuit which relies on knowledge of the location of the colorstripe burst (for the case of the color stripe burst video linelocations being stationary) generates vertical and horizontal timingsignals from the input video signal and from these generates indicatorsignals indicating particular video lines in each video field andparticular portions of the line for which it is desired to provide amodification to the color burst.

The detector approach typically uses a phase detector which includes asubcarrier regeneration circuit such as a phase lock loop, crystalfilter or frequency multiplier circuit for determining the phase of thecolor burst, and then compares this detected phase to the nominal phase(using a phase comparator) and provides an indicator signal when thecolor stripe burst is present, i.e. when the color burst phase has beenmodulated to deviate from the normal (correct) phase. This indicatorsignal then controls the desired modification to that line.

The actual modifications to the video lines to defeat the color stripeprocess fall into two main categories. In the first category, the colorburst itself is altered so as to defeat (eliminate or attenuate) itseffect on a typical VCR. In the second category, the horizontal syncpulse immediately preceding a color stripe burst is altered, thuscausing a VCR not to respond to the succeeding color stripe burst.

The first approach (in which the color stripe burst itself is modified)may be performed in several ways.

In one embodiment the color stripe burst is blanked out and notreplaced. Alternatively, the color stripe burst is blanked out and a newcolor burst of correct phase inserted therefor. The color stripe burstalternatively is phase shifted so that it is corrected in phase.

In another embodiment, the color stripe burst is delayed in time withrespect to the trailing edge of the preceding horizontal sync pulse, sothat the color stripe burst occurs outside the detection window of theVCR color burst circuitry.

In another embodiment, the phase error (shift) present in the colorstripe burst is measured and a color burst of a negative vector phase isadded thereto, and then the summed color burst may be attenuated to anormal level. In another embodiment, a very large amplitude color burstof correct phase is added at the color stripe burst location and thenthe summed color bursts are attenuated to a normal level, thuseffectively eliminating the effect of the color stripe burst. (This canbe done on all video lines—those having color stripe bursts as well asnon-color stripe burst lines.)

In another embodiment, a color burst of inverted phase (opposite to thatof the color stripe burst) is added to substantially null out the colorstripe burst, and then a color burst of correct phase is added in. Inanother related embodiment, the correct phase of a normal color burst isdetermined and the difference between the phase of that normal colorburst and the phase of the color stripe burst is measured. Then a signalwith a negative phase as to the difference is generated and used tomodify the color stripe burst, to produce color bursts that are“swinging” in phase the same amount of degrees from the correct burst inalternate lines. That is, in an example, the color stripe burst phase is+45°. Then one modifies a sufficient number of TV lines with each colorstripe burst having about half of the color burst having a phase of+90°, while changing the other half of the color burst to have a phaseof −90° (the opposite phase angle). This swinging is averaged out by atypical VCR.

In another embodiment, all the color bursts of correct phase throughoutthe TV field are replaced with color bursts having the phase of thecolor stripe bursts or having some (arbitrary) chroma phase angle. Thenthe chroma phase of each corresponding TV horizontal line is modified tomatch the phase of the color stripe bursts or to be the arbitrary chromaphase angle.

It is to be understood that in each of these embodiments it has beenfound that it is not necessary to modify all of a particular colorstripe burst; it has been found that modifying as little as one-half ofa color stripe burst effectively eliminates its effect for a typicalVCR. Also, it has been found that it is not necessary to modify eachcolor stripe burst present in a video field; the typical VCR can make acopiable recording of a signal with as much as one-half of the originalcolor stripe bursts still present for a typical commercial embodiment ofthe color stripe system.

Another embodiment which involves modifications to the color stripeburst itself heterodynes the color stripe burst signal into the correctphase. One can also use heterodyning to effectively blank out burst asseen by the VCR. This results for instance in heterodyning the colorstripe burst to a new frequency to which the VCR will be non-responsive,i.e. a frequency substantially higher or substantially lower than thenormal TV subcarrier frequency.

The other broad category of methods for defeating the color stripeprocess uses various modifications to the horizontal sync pulsespreceding at least some locations of the color stripe bursts. If the VCRsync separator fails to sense a horizontal sync pulse, the VCR will notgenerate a burst sampling pulse and hence will not detect any subsequentcolor stripe burst. Thus it has been found that removing the horizontalsync pulse of a color stripe burst line results in a copiable videosignal. Additionally, it has been found that actual removal of thehorizontal sync pulse is not necessary; instead the horizontal syncpulse may be for instance attenuated in width (duration) by narrowing itto the point where the horizontal sync pulse is not detected by thecorresponding sync separator circuitry in the VCR, and hence thesucceeding color stripe burst is also not detected.

Also, it has been found that removing only some of the horizontal syncpulses coincident with the color stripe bursts reduces the effectivenessof the color stripe process. Other embodiments for modifying thehorizontal sync pulse include DC level shifting upward of the horizontalsync pulse, thus causing the VCR sync separator to fail to produce anoutput signal.

Another embodiment relating to the horizontal sync pulses is to blackclip or amplitude attenuate the horizontal sync pulses so that they arenot sensed by the VCR's sync separator.

Another embodiment which also relates to the horizontal sync pulses addsa delay of about 2 microseconds between the trailing edge of thehorizontal sync pulse and the beginning of the color stripe burst; thusthe VCR color burst sampling pulse missamples the delayed color stripeburst and misses it.

It is to be understood that while the description herein generallyrefers to NTSC TV, with relatively minor modifications of the type wellknown to one of ordinary skill in the art the methods and circuitsdescribed herein are suitable for use with PAL TV, which similar to NTSCTV, has a color burst immediately following a horizontal sync pulse. Themajor differences between NTSC and PAL television, i.e. the number oflines per field and number of fields per second, are not material to thepresent invention and is the circuits described herein may be readilymodified to accommodate PAL TV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a standard NTSC TV waveform.

FIG. 1C shows a modification to the waveform of FIG. 1B therebyillustrating the color stripe process.

FIGS. 2A to 2G show waveforms illustrating various ways of defeating thecolor stripe process in accordance with the invention.

FIG. 3 shows a block diagram of an apparatus in accordance with theinvention.

FIGS. 4A to 4C show circuits for generating a correct color subcarrierfrequency and other signals to defeat the color stripe process.

FIGS. 5A and 5B and 6A and 6B show various circuits to defeat the colorstripe process.

FIG. 7 shows a circuit to improve playability, for use with eliminationof horizontal sync pulses.

FIG. 8 shows another circuit to defeat the color stripe process.

FIG. 9 shows a circuit for subcarrier regeneration.

DETAILED DESCRIPTION

The following describes a number of embodiments to defeat the colorstripe process. First is a description relating to waveforms andprocesses; second is a description of various related circuits.

Process Description

The following are various color stripe defeat processes in accordancewith the invention.

1. One or more color burst phase lock loops (or other circuits) are usedto find the mean color burst phase and then all color bursts (whethercolor stripe or not) are replaced throughout the video signal.

This replacement may be of only a portion of a particular color burst.For instance, of the standard eight to ten cycles of NTSC color burst,one may replace e.g. the first five cycles, the last five cycles, or anyother group of e.g. four to six cycles. The replaced cycles need not beconsecutive; one may replace alternate cycles, leaving “good”(corrected) cycles interspersed with “bad” (color stripe) cycles. Italso is possible to add corrected color burst cycles outside of theirnormal location and overlying the horizontal sync pulses, since thesewill be detected by a VCR. It is to be understood that the recognitionby the present inventors that only a portion of a particular color burstneed be replaced forms a part of the invention. Moreover, the partialreplacement is also applicable to other of the embodiments describedhereinafter.

2. A crystal or horizontal line phase locked loop (or equivalent such asa burst crystal filter) supplies a signal whose frequency is a 455 or910 (in NTSC TV) multiple of the horizontal line frequency and divideddown to the color frequency, with phase reset of every field based onodd or even field identification. This color frequency is used toreplace (in the sense described above) all or sufficient of the colorstripe bursts to allow a copiable result.

3. A color phase lock loop is used to identify the specific horizontalvideo lines that are color striped, and then the color stripe burst isphase shifted (using e.g. a conventional phase shifter circuit) toobtain a usable recording.

4. Determine which are the color stripe horizontal lines, and coincidentwith these color stripe lines, switches in a phase shifted color burstto replace the color stripe bursts or all bursts.

5. Either sense the color stripe video lines (e.g. via a color phaselock loop) or identify color stripe video lines otherwise and delay eachactive video line and thus the chroma so as to provide a copiablesignal.

6. Either sense the color stripe lines (e.g. via a color phase lockloop) or identify color stripe lines otherwise, and phase shift thechroma in the active lines where the color stripe process is located tomake a copiable color stripe free tape. Phase shifting the chroma isaccomplished by a conventional circuit such as an operational amplifierhaving its inverting terminal connected via a resistor to the inputsignal and its non-inverting terminal connected via a capacitor to theinput signal. The input terminal of its operational amplifier isconnected to the inverting terminal via a resistor, and also a resistorof the same value is connected to the inverting input terminal of theoperational amplifier and its output.

7. Locate video lines in which the color stripe bursts occur, thenmeasure the color stripe burst phase error. This is done with a phasedetector with the color stripe burst as one input and a color subcarrierregeneration circuit (e.g. a color crystal ringing circuit) supplyingthe second input. Then a color burst of a negative vector phase is addedto correct the burst phase and (in one version) readjust the burstamplitude to a normal level of e.g. 40 IRE units.

8. Locate video lines in which the color stripe bursts occur, theneffectively replace the color stripe bursts by adding a very largeamplitude of the correct color burst to the color stripe burst and thenattenuate the resultant summed burst, thus effectively eliminating anyeffect of the incorrect color stripe).

9. Replace the color stripe bursts by first locating the color stripebursts, then adding color bursts of an inverted phase to substantiallynull out the color stripe bursts and adding in color bursts of correctphase. This requires ascertaining the phase of the color stripe bursts,by observation or by measurement as above. This process results inreplacement of the color stripe bursts, without having to switch thebursts out.

10. Use a color frequency phase lock loop (or other method) to find thecorrect phase of a normal color burst signal and to find the phasedifference between the normal color burst and the color stripe burstsignals. Using this information, generate a signal with a negative phaseof this difference and use this signal to modify all or part of thecolor stripe burst with the negative phase burst signal to produce colorbursts that are swinging in phase the same amount of degrees from thecorrect burst from one line to the next line or within the portions ofburst in the same lines. A VCR tends to average the TV line to next lineswinging plus or minus phase burst signals, and/or the plus/minus burstphase portions within each TV line. Therefore the resulting signal tendsto produce less hue errors than does the unmodified color stripe signal.

Variants of the above described methods are illustrated in FIG. 2A andfollowing. FIG. 2A illustrates (in simplified form) the horizontal syncpulse and color stripe burst of FIG. 1C. The hatched area is the colorstripe burst area; the individual color burst cycles are not shown herefor simplicity. In the case of FIG. 2A, the entire color burst is phaseshifted as indicated by the hatching.

One method to defeat the color stripe process is illustrated in FIG. 2B,wherein a portion (here the right hand or later portion) of the colorstripe burst is modified to be in correct phase and/or negative colorstripe burst phase, as illustrated by the absence of hatching. Asdescribed above, it has been found that if about half or more of theduration of the color stripe burst has its phase corrected or isblanked, the color stripe burst is effectively defeated. That is, theNSTC color burst is 8 to 10 cycles long; it has been found thatmodifying 4 to 6 of these cycles is adequate.

FIG. 2C shows another method to defeat the color stripe process; herethe central portion of the color stripe burst is blanked. The first andsecond portions are corrected to the correct phase; this shows that theentire color stripe burst need not be present in order to achieve propercolor functioning of the VCR and/or television set.

FIG. 2D shows a variant of FIGS. 2B and 2C, where the first and lastportions of the color stripe burst have their phase corrected, but thecentral portion remains with the incorrect phase. As shown hereapproximately 30% or 40% of the color burst remains at the incorrectphase but this still effectively defeats the color stripe process.

In FIG. 2E the entire color stripe burst has been blanked with nosubstitution provided. In this case it has been found that there is noneed for a color burst in each and every video line for effectivefunctioning of most television sets and VCRs.

In FIG. 2F the first portion of the color stripe burst has been blanked;instead a few cycles of color burst of correct phase are overlaid on theactual horizontal sync pulse. Even at this location they will bedetected by the color synchronization circuitry of the TV set or VCR. Aportion of the color stripe burst is still present i.e. the centralportion; the last portion of the color stripe burst has its phasecorrected to be normal.

FIG. 2G shows a last and obvious variant wherein the entire color stripeburst has its phase corrected, by replacing or altering the color stripeburst to be of correct phase.

Other defeat methods include those that relate to the horizontal syncpulses:

11. Replace all correct phase color bursts throughout the TV field withcolor bursts having the phase of the color stripe bursts. Then modifyeach corresponding active horizontal TV line's chroma phase to equalthat of the color stripe bursts. For example, if the color stripe burstphase is 180° off from the correct color burst phase, then one modifiesthe correct color burst phase color burst by 180°. One can also replaceall color bursts with bursts having an arbitrary phase, and then phaseshift the chroma phase in the active portions of the TV lines to beequal to the arbitrary phase.

This modifying can be done by phase shifting and/or delay circuitsand/or inverting amplifiers. With this modification, the chroma phase ineach TV horizontal line is 180° off from that of the modified burst asdescribed above. To correct this discrepancy, one then modifies each ofthe TV horizontal active lines by phase shifting its chroma by 180°.This can be done by switching in a phase shifted or delayed version ofthe original video active horizontal TV lines in conjunction with themodified color burst. Again one can obtain a copiable recording bymodifying a sufficient number of correct color bursts and phase shiftinga sufficient number of TV horizontal lines.

12. Remove the horizontal sync pulse such that the VCR burst detectioncircuit (which usually relies upon a preceding horizontal sync pulse) isdisabled. One can effectively “remove” horizontal sync pulses severalways. For instance, one can remove the horizontal sync pulsesimmediately preceding at least some locations of the color stripe burst.It has been found that removing e.g. four horizontal sync pulsescoincident with a band of color stripe burst video lines results in acopiable recording without adversely affecting the horizontal linetiming of the VCR. Removal of these horizontal sync pulses can also bedone by narrowing the horizontal sync pulse coincident with the colorstripe burst lines. This narrowing is done until the color stripeeffectiveness is reduced to the point it is possible to make a copiablerecording. As an example, one may reduce the horizontal sync pulses downto 100 nanoseconds width. It has also been found that removing only someof the horizontal sync pulses coincident with the color stripe burstlines reduces the effectiveness of the color stripe signal. For example,every other line where a color striped burst occurs, the horizontal syncpulse is removed from that line.

13. DC level shift upward the horizontal sync pulses preceding the colorstriped bursts. This causes the VCR's sync separator to fail to producean output in response to these level-shifted horizontal pulses.

Other methods to effectively remove the horizontal sync pulses are toblack clip or amplitude attenuate them to about 20 IRE or less so thatthe VCR's sync separator will not sense these smaller amplitudehorizontal sync pulses, and thus not create a burst sampling pulse, whencolor stripe bursts are present.

These last two methods may lead to “playability” problems, due to themissing horizontal sync pulses. “Playability” refers to the resultingvideo signal including significant visual defects due to “slicing” offof active video as caused by improper horizontal sync pulse separation.This causes some sync separators in televisions receivers or VCRs togenerate false horizontal synchronizing pulses. To minimize suchplayability problems one can:

Add a pedestal voltage or signal such as a ramp of 0 IRE at thebeginning of the active TV line to about 10 IRE at the end of the TVline to all active TV lines; and/or

widen all other horizontal sync pulsewidths to about 6 microseconds.

14. In video lines where color stripe bursts are present, add a delay ofabout 2 microseconds or more such that the VCR's burst sampling pulse(triggered by the horizontal sync pulse) between the trailing edge ofthe horizontal sync pulse and the beginning of the color strip burstmissamples (misses) the delayed color stripe burst.

15. Heterodyne the color stripe burst signal into the correct phase bymixing it with a signal such that the resultant has the correct phase.

16. Heterodyne the color stripe burst to a new frequency such that theVCR will be non-responsive to it. For example, the color stripe burstcould be shifted by heterodyning it to a 15 MHz signal or a 2 MHzsignal.

In any of the above embodiments, one may replace the color phase lockloop with at least one stage of crystal filtering such as a ringingcircuit.

General Circuit

FIG. 3 shows a block diagram of an apparatus in accordance with thepresent invention suitable for carrying out the above-described methodsfor defeating the color stripe process. An input video signal “video in”is typically provided from a cable television source, but possibly fromother sources such as prerecorded video tape. (However, the color stripeprocess is generally not suitable for pre-recorded video tape.) Theinput video signal is provided to a circuit including a color stripelocation memory 12. This is typically a programmed read only memory.e.g. an EPROM, which includes data indicating on which of the 525 linesof the NTSC television field the color stripe bursts are located. ThisEPROM is programmed prior to assembly of the circuit, and the knowledgeof the location of the color stripe burst is determined by observation.Therefore it might be determined that the color stripe pattern is thecommercial embodiment as described above with four video lines havingthe color stripe burst followed by eight video lines without the colorstripe burst, etc. The output signals of the color stripe location fromthe memory 12 include a line location gate (LLG) signal indicating onwhich lines the color burst is located. The LLG signal is thus high forthe entire duration of a line having a color stripe burst.

A second output signal from the color stripe location memory 12 is apixel location gate (PLG) signal which indicates in exactly whichportions of the color burst are to be modified. The LLG signal is usefulbecause as explained above in certain embodiments of the invention onlya portion of each color burst is modified and other portions are not.Thus typically the PLG signal is high for only a portion of a colorstripe burst, but it may of course be high for the entire duration of acolor stripe burst where it is desired to modify and/or eliminate theentire color stripe burst.

Again, the data to generate the PLG signal is stored in a part of memory12 which stores enough data to divide up the color stripe burst intoe.g. 20 segments and to modify or not modify each of those segments.Since the color burst in NTSC television is eight to ten cycles induration, each of these cycles may be treated individually by the PLGsignal.

An oscillator 16 provides an output signal having the subcarrierfrequency signal (3.58 MHz for NTSC). This oscillator (timing signalgenerator) may be for instance a phase lock loop, or a crystal filteroscillator, or may derive the subcarrier frequency from the frequency ofthe horizontal sync pulse edges and then multiply the horizontal syncpulse frequency by frequency multipliers or by a phase lock loop tocause the circuit to lock on to the correct color subcarrier frequency.

A phase detector circuit 18 provides an output signal which is eitherlogically high or low and is called the color stripe detector (CSD)signal. Thus when the signal is high it indicates that the color stripeburst has been detected in a particular video line. This CSD detectionsignal is useful when the video line locations of the color stripebursts are not known. This is typically used where the color stripeburst locations are dynamic, i.e. not stationary. Thus use of the phasedetector is an alternative to the use of the LLG signal and they aretypically not both used in a single circuit. Thus the circuit of FIG. 3is a generalized representation of several alternative circuits sharingcommon elements and shown here as one circuit for purposes ofexplanation.

The phase detector circuit 18 includes a phase detector, the outputsignal of which is provided to a comparator to compare the phase of aparticular color burst to that of a normal color burst. If thecomparison indicates no difference, then the color stripe detectorsignal is low, i.e. there is no color stripe burst present; if there isa difference, then the color stripe detector signal is a high signalindicating the presence of a color stripe burst.

The right hand portion of FIG. 3 shows a generic modification circuit22. This circuit 22 may be any one of a number of circuits, each ofwhich performs one of the types of modification to the video signal asdisclosed above and is described in detail hereinafter. In addition toreceiving as inputs the indication of the presence of the color stripe,i.e. either the LLG signal or the CSD signal, and the PLG signalindicating which portions of the color stripe are to be modified as wellas the subcarrier frequency signal, the modification circuit alsoreceives the input video signal.

The output signal of the modification circuit 22 is a video signal“video out” which is free of (or has only an attenuated) color stripeprocess and hence is copiable by a typical commercially available VCR.

As described further below, the modification circuit either attenuatesor eliminates the color stripe process by direct modification of thecolor stripe burst or alternatively operates on the horizontal syncpulse immediately preceding a color stripe burst, and by modifying thehorizontal sync pulse causes the VCR to ignore the ensuing color stripeburst.

Therefore by logically ANDing the output of the phase detector (the CSDsignal) or the LLG signal with the PLG signal, one may select whichvideo lines to modify and which portion of each line is to be modified.As described above, it has been found generally that it is adequate forinstance to modify as little as half of a particular color stripe burstin order to defeat the color stripe process as regards that color stripeburst. Moreover, it has been found experimentally that for typicalcommercial embodiments of the color stripe process one may modify oreliminate as few as one half of the color stripe bursts and stilleffectively defeat the color stripe process, i.e. provide a copiablevideo signal. It is to be understood that herein the terms “copiable”and “recordable” both mean that the resulting video signal, whenrecorded by a VCR and then played back, provides a viewable televisionpicture without substantial hue defects due to the color stripe process.Thus these terms refer to effective elimination (defeat) of the effectof the color stripe process in terms of viewability of the video signal.

Exemplary Circuits

FIGS. 4A, 4B, 4C illustrate several exemplary circuits for generatingthe correct color subcarrier frequency and other signals to be used toreplace the color burst signal component in the output video of thedevice. FIG. 4C also illustrates a circuit for generating timing signalsto defeat the color stripe process to provide a copiable video signaltherefrom. Thus FIGS. 4A, 4B, 4C and FIGS. 5A, 5B, 6A, and 6B showvarious particular versions of the circuit of FIG. 3.

Copy protected video from a video source (such as cable television) isconventionally demodulated (not shown) to produce baseband video usingwell known techniques. This copy protected video usually contains stablevideo with horizontal and vertical sync and subcarrier coherency, andincludes the color stripe process as described above. The copyprotection may also include pseudo-sync and AGC pulse pairs as describedin the above-referenced U.S. Pat. No. 4,631,603 to Ryan and raised backporch pulses as described in U.S. Pat. No. 4,819,098 also to John O.Ryan and incorporated herein by reference. These pseudosync and AGCpulse pairs may be removed using the techniques described in U.S. Pat.No. 4,695,901 to John O. Ryan and also incorporated herein by reference.Also incorporated by reference are U.S. Pat. No. 5,194,965 to Quan etal., U.S. Pat. No. 5,157,510 to Quan et al., and U.S. patent applicationSer. No. 08/062,866 filed by Wonfor et al. which also disclose copyprotection and defeat techniques relevant to the present invention.

This baseband video in signal (see FIG. 4A) is input to amplifiers A1and A2. The output of amplifier A1 is coupled to sync separator U1 whichis e.g. National Semiconductor Corp. part number LM1881 or equivalent.Sync separator U1 generates a frame pulse on line 16, a horizontal syncpulse on line 18, and a burst gate signal on line 20. Amplifier A2operates as a sync tip clamping amplifier. One-shot U89 generates a syncpulse sample pulse to cause amplifier A2 to be clamped at sync pulses toa specific voltage, i.e. −40 IRE.

Sync tip clamping is desirable since the input signal may include theabove described pseudo-sync and AGC pulses which would cause back porchclamp circuits to behave incorrectly. The output signal of Amplifier A2is about 1 volt peak-to-peak and the blanking level of the video outputsignal of amplifier A2 is clamped to about zero volts. Burst gateinverter U20 is coupled to the control terminal of switch SW1. Theclamped video signal from amplifier A2 is coupled to a first input ofswitch SW1 and to the “Clamped Video” output line of FIG. 4C. The secondinput terminal of SWQ is coupled to ground.

Switch SW1 gates out the color burst portion of the input video toproduce a gated color burst signal on line 30. Chroma amplifier A3amplifies the gated color burst signal on line 30, and its outputterminal 34 is coupled to a first input terminal of AND gate U100. Theother input terminal of gate U100 is connected to the DS Output terminalof EPROM U5 (see FIG. 4B). EPROM U5 38 is a 525 line EPROM discussedmore fully later; its D5 output terminal provides a signal that istypically high during the active field and low during the verticalblanking interval. It is necessary for the signal at terminal D5 to be“on” during the entire active TV field since it can be programmed to beon during the time of normal color burst signal and/or be low during thetime during the time of color stripe bursts and/or low during verticalblanking interval (VBI) lines without color burst. The output signalfrom gate U100 on line 42 is bursts from the input video, with possiblerestrictions to particular lines in the VBI.

Color burst phase lock loop U2 has a slow and long time constant in itsDC amplifier such that its output signal is phase constant, even thoughcolor stripe bursts with incorrect phase burst signals are present inthe video input 42. PLL U2 may alternatively may be a crystal burstcontinuation oscillator that is injection locked, such as the RCACA1398, or a burst ringing circuit such as a crystal filter. The outputterminal 46 of PLL U2 is coupled to a phase shifter ø2. The outputsignal of phase shifter ø2, on line 50 is a stable 3.58 MHz sine waveCW2 that is used in the circuitry of FIGS. 5A, 5B to produce recordablevideo signals.

The above discussion assumes that the locations of the color stripebursts are known and fixed. If the location of the color stripe burstsis moving in line location over time, these color bursts instead aredetected on a line-by-line basis. This detection is done by comparingthe input video burst signal to gate U100 with the output signal ofcircuit U2 using phase detector (PD) U11 (see FIG. 4C). The outputsignal of phase detector U11 is coupled to 3 MHz low pass filter (LPF)LPF3 to an input terminal of switch SW20. Switch SW20 is controlled byan inverse clamp pulse on line 24. Switch SW20 and capacitor C4 sampleand hold the output of the 3 Mhz low pass filter LPF3 during the burstgate interval. The non-inverting input signal to amplifier A4 hence is aline-by-line identification of the burst phase. An incorrect burst phasefrom a color stripe signal causes a different voltage to appear at theinput terminal of amplifier A4 than when the correct burst phase occurs.Amplifier A4 operates a threshold detector that triggers high when colorstripe bursts are present. The output signal of amplifier A4 hence is acolor stripe detection signal (CSD) on line 62 that is used in thecircuitry of FIGS. 5A, 5B to produce copiable signals.

A second method to generate a stable color subcarrier is to derive asubcarrier signal from the horizontal sync portion of the video signal.This method is applicable when there is sync-to-burst coherence, whichis the case when the color stripe process is applied in cable televisionapplications. To achieve this, a horizontal sync pulse signal on line 18from sync separator U1 is coupled to a 45 microsecond non-retriggerableone shot U3 to eliminate the horizontal sync pulses present during thevertical interval and/or the pseudo sync pulses when both are in thevideo input signal.

The output terminal of one shot U3 (see FIG. 4B) is coupled to a firstinput terminal of a 10 bit counter U4. The reset input terminal ofcounter U4 is coupled to a frame pulse output terminal of sync separatorU1 by way of line 16 and a differentiation circuit including capacitorC1 and resistor R1. The output signals of 10 bit counter U4 feed a 10bit bus to EPROM circuit US. EPROM US is programmed to set high or lowlogic levels within the TV frame (525 lines in NTSC). The outputterminals of EPROM US are D0 to D7. The signal on terminal D3 can behigh all the time or high during a portion of the TV field. The signalon terminal D3 controls the tri-state control of the flip-flop U6 Qoutput. One shot U3's output terminal is coupled to the clock inputterminal of phase detector flip-flop U6. Horizontal sync edges from oneshot U3 set flip flop U6, while the output signal of divider U7 resetsflip flop U6. The output terminal of flip flop U6 is coupled to low passfilter and amplifier LPF1 for filtering and amplification (see FIG. 4C).The output of filter LPF1 is coupled to a crystal voltage controlledoscillator VCO operating at 14.318180 MHz.

As a result of the input signal from filter LPF1, oscillator VCO islocked to the video horizontal sync pulses. The output terminal ofoscillator VCO is coupled to divide-by-4 counter U8 to produce asubcarrier frequency signal on line 76. The divide-by-4 counter U8 isreset at its CLR terminal every frame and results in a 0° or 180°ambiguity in the correct phase of subcarrier frequency on line 76. Tocorrect this, the signal of divide-by-4 counter U8 is phase compared byphase detector U10 with the normal burst of the input by sampling at avideo line known to have a normal burst, i.e. video line 14, and holdingvia capacitor C3. If the phase is correct from counter U8, amplifier A5outputs a low state and the output of exclusive OR gate U9 will notinvert the phase of the output signal of counter U8. If the phase of theoutput signal of counter U8 is incorrect (180°), phase detector (PD) U10supplies a voltage (via low pass filter LPF2 and switch SW10) toamplifier A5 such that the output of A5 amplifier is high. This thencauses the output of XOR gate U9 to invert phase by 180°.

Switch SW10 is controlled by the signal at terminal D6 of EPROM U5.

This circuit generates the correct subcarrier phase at the outputterminal of XOR gate U9. The output signal of XOR gate U9 is coupled tophase shifter ø1. The output of ø1 is a 3.58 MHz subcarrier signal CW1on line 94 that is used in the circuitry of FIGS. 5A, 5B to generate oneor more recordable output signals.

Additional circuitry in FIG. 4C includes 10 bit counter U60, the outputterminals of which are coupled to horizontal line pixel location EPROMU70. Individual pixels are located by resetting the 10 bit counter U60with horizontal rate edge signals from the output terminal of one shotU3 and clocking 10 bit counter U60 with the output signal of oscillatorVCO. The 10 bit bus output signals of 10 bit counter U60 are coupled tothe address lines of EPROM U70 to generate outputs DD0 TO DD7. Theseoutputs (pixel location gate signals) represent pixel locations withinthe horizontal lines throughout the video field.

In addition to the FIG. 4C output signals discussed above, severaladditional output signals from FIG. 4C are used in the circuitry ofFIGS. 5A, 5B. A first group of these include: (1) the D0 output signalof EPROM U5 which provides an “all locations” indication of color stripepulses designated ACSL on line 100; (2) the D1 output signal of EPROM U5which provides a “some location” indication of color stripe pulsesdesignated SCSL on line 102; and (3) the output signal at terminal D4 ofEPROM U5 which provides an “all active field” output designated AFL online 104. These signals correspond to the line location gate signal ofFIG. 3.

Additionally, there is: (1) a horizontal sync HSYNC output signalprovided on line 18 by the horizontal sync output terminal of syncseparator U1; (2) a CLAMPED VIDEO output signal on line 108 provided bythe output terminal of clamp amplifier A2; and (3) a BURST GATE outputsignal provided by a burst gate output terminal 20 of sync separator U1.All of these output signals are used for various parts of the circuitrydescribed in FIGS. 5A, 5B, 6A, 6B.

FIGS. 5A, 5B and 6A, 6B show various exemplary circuits to use the colorsubcarrier and other signals generated in FIG. 4C for various methods toproduce a video output signal recordable by a videocassette recorder.Hence FIGS. 5A, 5B and 6A, 6B are illustrative of various possiblecircuits; an actual circuit hence would only include selected portionsof what is shown in FIGS. 5A, 5B and 6A, 6B.

The first of these circuits produces a copiable video output signalVIDOUT 1, at terminal 200. The user can select a suitable subcarriersignal generated in FIG. 4C by selecting either the signal CW1 on lines94 or signal CW2 on line 62 using a jumper JP1. The output terminal ofjumper JP1 is coupled to an attenuator PAD which attenuates the selectedsubcarrier signal. Clamped video on line 108 and attenuated subcarrierfrom the attenuator PAD are coupled to the first and second inputs ofswitch SW100. Switch SW100 is controlled by an output terminal of ANDGate U305, with one input terminal of gate U305 coupled to output lineDDo of EPROM U5 carrying a burst gate signal of a width depending of theprogramming of the EPROM U5. The other input terminal of AND gate U305is selectively connected to the ACSL signal on line 100, the SCSL signalon line 102 or the CSD signal on line 62 using a combination of jumperJP2 and jumper JP3.

This circuit permits the user to select video lines of the recordablevideo output VIDOUT 1 at terminal 200 which are to receive replacementcolor burst signals. If the ACSL signal is selected, the recordablevideo output VIDOUT 1 includes corrected color bursts on all video lineswhere color stripe burst are known to be. If the SCSL signal isselected, the recordable video output VIDOUT 1 includes corrected colorbursts on a sufficient number of video lines to substantially reduce ornullify the effects of the color strip process on the recorded video.

The ACSL signal is preprogrammed and indicates the video lines which asdetermined by observation have in turn been preprogrammed by the colorstripe generator. (The color stripe generator is the apparatus, notillustrated here, that puts the color stripe process into the videosignal.) In some cases it is not known which lines have beenpreprogrammed, and then the CSD Signal is used to determine which videolines need to have color burst corrected. If the CSD signal is selectedusing jumper JP3 to drive AND Gate U305, the circuit replaces all or atleast a majority of the color stripe burst in the recordable videooutput VIDOUT 1, such that the effects of the color stripe bursts areessentially nullified.

In each of the above techniques, the signal on the DDo output terminalof EPROM U5 is programmed to switch in enough of a portion of thecorrect burst in each line to substantially reduce or nullify the colorstripe process.

A second circuit produces a recordable video output signal VIDOUT 2, atterminal 214. This circuit switches in a phase shifter during the videolines where a color stripe burst is known, to shift the known phaseerror of the color stripe burst. A first input signal to switch SW102 isclamped video on line 108 which contains color stripe bursts with aknown value of phase shifted color burst signals. The second inputsignal to switch SW102 is the output signal of phase shifter ø3, whichis a phase shifted version of the clamped video signal. The controlterminal of switch SW102 is connected to the output terminal of AND gateU305 which provides the same switching pulses as does control switchSW100. These pulses permit selection of enough of a corrected colorburst of essentially the correct phase in each line to substantiallyreduce or nullify the color stripe process.

A third circuit for providing a copiable video output signal adds in alarge amplitude of a correct burst signal to the color striped video andthen attenuates the resultant burst to nominal burst levels. Thiscircuit is accomplished as follows.

The clamped video signal on line 108 is coupled to a first inputterminal of summing amplifier A36. The color subcarrier signal selectedby jumper JP1, either signal CW1 or CW2, via attenuator PAD is coupledto an input terminal of switch SW101. Switch SW101 selects theattenuated color subcarrier, that is at normal burst amplitude, duringthe times determined by EPROM U305 as discussed above. The outputterminal of switch SW101 is coupled to amplifier A35, which is a 10×amplifier producing a color subcarrier burst signal 10 times the normalamplitude.

This amplified color burst signal is coupled to a second input terminalof summing amplifier A36, where it is summed with the clamped videosignal on line 108 containing color stripe bursts. The output terminalof summing amplifier A36 is coupled to a switched attenuator includingresistor R9, resistor R10 and switch SW104. Switch SW104 is controlledby the burst gate signal from sync separator U1 at line 20 on line 110.Closing switch SW104 230 attenuates the output signal of amplifier A36during the duration of the burst gate signal. This in effect “swamps”any color stripe bursts present in the input video signal. The output ofthe switched attenuator is coupled to amplifier A44, which is a unitygain amplifier, to provide copiable video output VIDOUT 3 at terminal218. Note that switch SW104 which causes the burst amplitude reductioncan be closed during some or all of the color stripe lines and alsoduring a portion of the color burst in each line to produce a recordable(copiable) signal. Note also that the addition of a large amplitudecolor burst via amplifier A36 followed by burst amplitude reduction canbe done a majority of the video lines in a video field to produce arecordable signal.

A fourth technique for providing a copiable video signal includesremoving the color stripe burst and/or the horizontal sync pulsespreceding the color stripe bursts. By doing this, the recording VCR willnot try to heterodyne correct the color stripe burst line with anincorrect phase burst signal. All or some of the lines with color stripebursts that have horizontal sync pulses or color stripe bursts blankedout results in a recordable copy. It should be noted that only some ofthe relevant horizontal sync pulses are narrowed or only some of therelevant color stripe bursts are narrowed to produced a recordableoutput.

A circuit for implementing this fourth technique is as follows. Clampedvideo on line 108 is coupled to resistor R107 which in turn is coupledto unity gain amplifier A55. A combination of NAND Gate U110, AND GateU120, jumper JP5, jumper JP4, and OR Gate U130 provides the timingsignals to blank out color stripe burst and/or the horizontal syncpulses preceding the color stripe bursts. Switch SW103 grounds the inputterminal of unity gain amplifier A55 whenever the selected pulses orburst intervals are selected by the elements outlined above. The timingcomponents described above may blank out the horizontal pulses or colorstripe burst signals during all or only during a portion of a colorstripe burst period or its accompanying horizontal sync pulse. Theoutput terminal of unity gain amplifier A55 provides recordable videooutput VIDOUT 4 at terminal 220.

FIGS. 6A, 6B illustrate other circuits to use the color subcarrier andpulses generated by the circuitry of FIG. 4C to produce recordable(copiable) video output signals.

A fifth technique for producing a recordable video signal eliminates theeffect of the horizontal sync pulses associated with the color stripebursts, using level shifting the horizontal sync pulses. The effects oflevel shifting are described in “Method and Apparatus For DisplayingAnti-Copy Protection In Video Signals”, U.S. Pat. No. 5,194,965 issuedto Quan et al. on Mar. 16, 1993, and which is incorporated by reference.

This fifth technique is accomplished as follows. Clamped video on line108 from FIG. 4C is coupled to a first input terminal of summingamplifier A99. The other input terminal of amplifier A99 is connected toan output terminal of gate U120 which may contain a positive goinghorizontal sync pulse coincident with a color stripe burst. The outputterminal of gate U120 may also carry part of a horizontal sync pulsecoincident with some of the color stripe bursts. Which lines areaffected is a function of the timing described in the fourth techniqueabove. Thus the level shift of the horizontal sync pulse may occur onlyin a portion of a specific line or in the specific lines having colorbursts. The amount of level shifting may adjusted to produce the amountneeded to produce a recordable video output. The output of terminalsumming amplifier A99 250 provides the recordable video output signalwith level shifted horizontal sync pulses.

A sixth techniques for producing a recordable video signal is toeliminate the effect of the horizontal sync pulses associated with thecolor stripe bursts by clipping the associated horizontal sync pulses.

The sixth technique is accomplished as follows. A sync clipping circuitincludes amplifier A91, transistor QBCL and resistor RS. Amplifier A91inverts and attenuates the logic level of the gate U120 output signaldescribed above. The output signal of amplifier A91 is typically aboutzero IRE to −10 IRE. When the clamped video signal is coupled throughresistor Rs, its horizontal sync pulses will be clipped to −10 IREduring a portion of or all the color stripe burst lines, depending uponthe logic level output of gate U120. In addition, each horizontal syncpulse may be clipped for its full duration or part of its duration. Theamount of clipping duration depends on the ability to make a recordablecopy. Amplifier A77 outputs the recordable video signal VIDOUT 6 withclipped horizontal synchronizing signals.

A seventh technique for producing a recordable video output is toeliminate the effect of the horizontal sync pulses associated with thecolor stripe bursts by widening those horizontal sync pulses.

This is accomplished as follows. The clamped video signal on line 108 iscoupled to a first input terminal of switch SW124. The second inputterminal of switch SW124 is coupled to a widened horizontal sync signalwhich is provided by the DD3 output terminal of EPROM U70. Switch SW124is controlled by the signal at the output terminal of AND Gate U123 thatANDs the horizontal blanking signal at terminal DD4 of EPROM U70 and theactive field lines output signal on line 64.

This seventh technique uses the output signal of gate U120 to controlswitch SW124. The resultant signal on any video lines determined to havecolor stripe burst signals is no color burst, because the widenedhorizontal sync eliminates the color burst. Unity gain amplifier A88couples the recordable video output with the widened horizontalsynchronizing signals to the VCR.

FIG. 7 shows a circuit to improve playability in conjunction withelimination of horizontal sync pulses, by adding an offset voltage tovideo that has the horizontal sync pulses eliminated. This added offsetvoltage allows the sync separator in a TV or VCR not to slice at videolevels caused by missing horizontal sync pulses.

To generate a voltage pedestal during the active lines of the activefield, the active horizontal line pixel locations indicated by a signalat terminal DD5 of EPROM U70 are logically combined by AND gate U467with the signal at terminal D4, which indicates the active field linelocations. The output of gate U467 is provided to a current mirrorincluding transistors Q_(A) and Q_(B) via resistor Rped. The collectorof transistor Q_(A) feeds a current mirror including transistors Q_(D)and Q_(C). The collector of transistors Q_(C) then injects a pedestalcurrent into resistor Rss to add a pedestal voltage to the clamped videoinput. The output signal (via a buffer amplifier A40) then feeds intothe various horizontal sync pulse clipping, shifting or blankingcircuits described herein.

An eighth technique for producing a recordable video output is to delaythe color stripe burst so as to be out of the range of the burstdetection circuitry, to effectively cause the color burst to “drop out.”

The technique is accomplished as follows. Clamped video on line 108 iscoupled to a chroma band pass filter including resistor Ro, inductor L4and capacitor C4 and to a first input terminal of switch SW123. Theoutput signal of the chroma pass filter is buffered and amplified byamplifier A98. The output terminal of amplifier A98 is coupled to delayline 276 which delays the chroma output of the band pass filter by e.g.2 microseconds. The output terminal of the delay line 276 is coupled toa second input terminal of switch SW123. Switch SW123 is controlled bythe output signal of AND Gate U278. Horizontal sync pulses from syncseparator U1 are coupled to the input terminal of one-shot U505 whichgenerates a 4 microsecond pulse triggered by the trailing edge of thehorizontal sync input signal. AND Gate U278 generates a control signalfor switch SW123 from a logical combination of the output signal ofone-shot U505 and the D1 terminal output signal of EPROM US which is asignal representing some locations of color stripe Lines (SCSL), asdescribed above.

The output signal of switch SW123 has a delayed color burst on videolines having color stripe bursts. The delayed color burst is notdetected by a VCR. Therefore the VCR is not responsive to the lineshaving a color stripe burst. Amplifier A97 buffers the output signal ofswitch SW123 and provides an output signal with delayed color stripebursts that is recordable.

A ninth technique for producing a recordable video signal uses signalmultiplying (heterodyning) to shift the color stripe burst phase to becorrect and/or to shift the color stripe bursts out of the frequencyrange of the burst detection circuitry, to effectively cause the VCRcolor burst to “drop out”.

Clamped video on line 108 is coupled to a first input terminal of signalmultiplier 282, the second input terminal of is connected to a 1 volt DCsignal most of the time, as controlled by signal SW122. To control thecolor stripe burst phase, switch SW122 couples via jumper JP207 thecos(2πf_(sc)t+ø) signal from FIG. 4C generated by frequency doubleamplifier A10 and phase shifter ø4, or to shift the color stripe burstout of frequency range, a cos(2π18.6×10⁶t) signal from any oscillatorsource is used to heterodyne the color stripe bursts. Switch SW122 iscontrolled by the output of AND gate U305.

Because of the 1 volt DC at switch SW122 and the control signal fromgate U305, the output signal of multiplier 282 is “transparent” (equalto the signal on line 108) during most of the time. During the time ofcolor stripe burst signals as determined by the output signal of gateU305 however, the output signal of multiplier 282 is equal to colorstripe frequency plus a corrected color burst phase angle orcos(2πf_(sc)t+ø_(A)) and three times the color burst frequency plusanother phase angle or cos(2π3f_(sc)t+ø_(B)). The output multiplier iscoupled to low pass filter LPF4. Low pass filter LPF4 has a cut offfrequency of about 5 MHz so that the three times color burst frequencyis eliminated. The output signal of low pass filter LPF4 is coupled toamplifier A74 that buffers the output signal of low pass filter LPF4 andprovides an output with color stripe burst that hence are recordable.

If the second input terminal of switch SW122 is coupled to an 18.6 MHzsine wave, and low pass filter LPF4 is designed to cut off at 16 MHz,the output signal of multiplier 282 during the color stripe burst time,as determined by the output of gate U305, will have burst frequencies ofabout 15 MHz and 22 MHz. Such a LPF4 will pass through the 15 MHz burstduring the color stripe times. When this signal is coupled to a VCR, theVCR's chroma input filter will be unresponsive to the 15 MHz since it isexpecting a 3.58 MHz burst (and filters out higher frequency). Thusduring the lines with color stripe bursts, the color stripe burst isdefeated.

FIG. 8 shows a circuit for carrying out the above described method ofreplacing correct phase color bursts with color stripe bursts and thenmodifying the chroma phase to that of the color stripe bursts. Theclamped video signal is provided to phase shifter U75, which shifts thephase by an amount equal to the difference between that of the colorstripe bursts and the correct color burst phase. Switch SW124,controlled by the burst gate signal, outputs the clamped video having ineach line of the TV field a color burst having the phase of the colorstripe burst. Similarly controlled switch U126 then in turn outputs theclamped video having each TV horizontal line phase shifted (includingthe active chroma) to match the phase shift of the color stripe burst,which signal is copiable.

FIG. 9 illustrates a circuit for subcarrier regeneration without the useof a phase lock loop or a voltage control oscillator, for use inconjunction with the above described circuitry in one embodiment of theinvention. The output signal from one shot U3 of FIG. 4B is provided toa 32 μsec one shot U60, the output signal of which is the equivalent ofthe horizontal line frequency i.e. a square wave with the horizontalline frequency. This signal is provided to a band pass filter BPF3 whichpasses a 13th harmonic of horizontal sync. Thus this signal which is 13times the horizontal frequency is fed to a limiter amplifier A47 whichin turn is connected to the input terminal of a bandpass filter BPF4which passes the seventh harmonic of the 13 times the horizontalfrequency. This frequency, which is seven times the 13th harmonic of thehorizontal frequency, is provided to a second limiter amplifier A48which in turn is connected to the input terminal of a band pass filterBPFG which passes a band of a fifth harmonic of seven times the 13thharmonic of the horizontal frequency, and which in turn is connected toanother limiter amplifier A50 which connects to the clock terminal of adivide by 2 counter U68. The noninverting Q output terminal of counterU68 provides a signal of 3.57954 MHz which of course is exactly thedesired subcarrier frequency for NTSC television. This signal in turn isthe first input to a color phase identification circuit U70 (similar tothat shown in FIG. 4C) which provides as an output signal thereof (inresponse to the frame pulse provided at the other input terminal) thedesired correct color phase and frequency subcarrier for each TV field.A similar scheme to regenerate color subcarrier can be done via verticalsync signals through frequency multipliers and/or (crystal) phase lockloop circuits.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art and are intended to fallwithin the scope of the appended claims.

1. A method of modifying a video signal having a copy protection signal,with improved playability of the thustly modified video signal in atelevision set while reducing the effectiveness of the copy protectionsignal in a video cassette recorder (VCR), the video signal including aplurality of video lines, each video line including a color burst havinga predetermined phase, the method comprising: modifying the phase ofless than an entire portion of selected color bursts to be other thanthe predetermined phase, to attenuate the effects of the copy protectionsignal on said television set so as to provide said improved playabilityof the modified video signal in the television set while reducing saideffectiveness of the copy protection signal in the VCR.
 2. The method ofclaim 1, wherein the step of modifying includes shifting thepredetermined phase by 180°.
 3. The method of claim 1, wherein the stepof modifying includes shifting the predetermined phase by at least 20°.4. The method of claim 1, wherein in each video field, at least one bandof video lines is subject to modifying a portion of the color burst,followed by a band of video lines which are not subject to modifying aportion of the color burst.
 5. The method of claim 1, wherein a modifiedportion precedes an unmodified portion of the color burst.
 6. The methodof claim 1, wherein an unmodified portion precedes a modified portion ofthe color burst.
 7. The method of claim 1, wherein a first unmodifiedportion precedes a modified portion and a second unmodified portionfollows a modified portion.
 8. The method of claim 1, wherein the colorburst has a predetermined duration, further comprising extending theduration of the color burst prior to modifying the phase.
 9. A method ofmodifying a video signal to achieve optimal color functioning of atelevision set, the video signal including a plurality of video lines,each video line including a color burst having a predetermined phase,wherein selected video lines include a color burst having an incorrectphase other than the predetermined phase, said color burst of incorrectphase defining a copy protection signal, the method comprising:determining a duration of a portion of the color burst having incorrectphase; and modifying the amplitude of a part of said portion of thecolor burst while maintaining the incorrect phase in a remaining portionof the color burst to achieve said optimal color functioning of atelevision set.
 10. The method of claim 9, wherein the step of modifyingincludes blanking said part of said portion of the color burst havingincorrect phase.
 11. A method of modifying a video signal formed of aplurality of video lines to improve the playability of the thuslymodified video signal in a television set, the video signal including acopy protection signal formed of color stripe bursts of incorrect phasein selected video lines of the plurality of video lines, the methodcomprising: determining the selected video lines having the color stripebursts of incorrect phase; and modifying the incorrect phase of aselected portion of selected bursts of the color stripe bursts whilemaintaining the incorrect phase in a remaining portion of the colorstripe bursts, to attenuate the effects of the copy protection signal ona television set while improving the playability of the modified videosignal in the television set.
 12. The method of claim 11, wherein thestep of modifying comprises: modifying the phase of less than one-halfof said selected portion of the selected color stripe bursts in lessthan one-half of the selected video lines to cause the attenuation ofthe effects of the copy protection signal on a television set whileproviding said improved playability.
 13. The method of claim 11, whereina band of video lines is subject to the step of modifying the effects ofsaid selected portion of incorrect phase of the color stripe burst,followed by a band of video lines which are not subject to the step ofmodifying.
 14. The method of claim 11, wherein a modified portionprecedes an unmodified portion of the color stripe burst.
 15. The methodof claim 11, wherein an unmodified portion precedes a modified portionof the color stripe burst.
 16. The method of claim 11, wherein a firstunmodified portion precedes a modified portion and a second unmodifiedportion follows a modified portion.